1. Field of the Invention
The present invention relates to a vibration gyroscope that detects angular velocity.
2. Description of Related Art
Rotating gyroscopes of the mechanical type have been used in the past as inertial navigation apparatuses for aircraft and vessels, but their large size and high cost make them difficult to build into small electronic equipment and small transport equipment.
In recent years, however, research with regard to making compact gyroscopes has progressed, and progress has been made in the area of a practically usable vibrating gyroscope in which a piezo-electric element resonator is caused to vibrate, another piezo-electric element resonator mounted thereonto rotating, the vibration caused by the resulting Coriolis force being used to detect the voltage that is generated. This is used in car navigation systems and in shake-detection apparatuses for video cameras.
A gyroscope of the past which uses a piezo-electric element will be described below.
FIG. 13 is a perspective view of a tuning fork type of vibrating gyro of the past.
A tuning fork type of vibrating gyro of the past will be described with reference being made to FIG. 13. The resonator 71 is made of a constant-resiliency metal such as "Elinvar" and has the structure of a compound tuning fork.
That is, the resonator 71 has joined onto the top part of the first beams 72 and 73 the second beams 74 and 75. The piezo-electric element driving section and drive electrode 76 are attached to the first beam 73.
While it is not shown in the drawing, in the same manner another piezo-electric element driving section and drive electrode are attached to the first beam 72.
The piezo-electric element detector and detection electrode 77 are attached to the second beam 75 and in the same type of detection section and detection electrode are attached to the second beam 74. In this structure, the direction in which a beam extends is taken as the Z-axis direction.
Next, the action of this structure will be described.
As a result of an AC voltage that is applied to the drive electrode 76, the first beams 72 and 73 exhibit a first bending vibration which displaces them to the left and to the right. In the description which follows, this will be called "intraplane vibration," since it is normally customary to consider the vibration of a tuning fork in a single plane to be the ideal case.
In response to this intraplane vibration, the second beams 74 and 75 that are joined to the first beams 72 and 73 exhibit intraplane vibration.
If the overall tuning fork is caused to rotate about the Z axis at an angular velocity of .omega., a Coriolis force Fc acts in a direction that is perpendicular to the intraplane vibration. This Coriolis force Fc can be expressed by the following equation. EQU Fc=2.multidot.M.multidot..omega..multidot.V
In the above equation, M is the mass of the first beams 72 and 73 or of the second beams 74 and 75 and V is the velocity of the vibration. In accordance with the Coriolis Fc, a second bending vibration is excited which has displacement in directions that are perpendicular to the intraplane vibration.
This will be called extraplanar vibration.
By detecting the AC voltage that is generated by this extraplanar vibration using the detection electrode 77 or 79, it is possible to calculate and know the angular velocity .omega..
However, a vibrating gyro of the past had the following problem. When supporting a resonator, to minimize the influence of the support on the resonator, the support is generally made at a position of the resonator that does not move during vibration, which is at a vibration node.
In the tuning fork configuration which is shown in FIG. 13, the node of the intraplanar vibration is at the base part and while there is almost no movement in this area, in the case of extraplanar vibration which is excited by Coriolis force, there is no part that does not move in accordance with the vibration. Therefore, regardless of the method of support, the support will influence the resonator.
In general a tuning fork type of resonator is supported in the middle of the base part, and whereas the cases of supporting in this part and not supporting in this part, there is almost no change in the resonant frequency for the intraplanar vibration, in the case of extraplanar vibration the resonant frequency is changed by several percent.
Therefore, the extraplanar vibration will change several percent in accordance with the type of support used. In this case, the alternating Coriolis force that has the intraplanar vibration resonant frequency excites extraplanar vibration, but the excitation efficiency exhibits dependency upon the extraplanar vibration resonant frequency.
If there is distance between the intraplanar vibration resonant frequency and the extraplanar vibration resonant frequency, it is not possible to achieve sufficient excitation of extraplanar vibration, and a small change in the support can make a large change in the extraplanar vibration resonant frequency, so that the excitation efficiency greatly changes, making detection with good accuracy impossible. For this reason, tuning fork type vibrating gyros did not enjoy sufficiently wide use.
At present, there have been proposed various vibrating gyros having configurations such as a tuning fork configuration or a single-beam configuration, and because a vibrating gyro detects a Coriolis force that acts in a direction that is perpendicular to the vibration direction, it is thought to be advantageous to have a configuration that has symmetry about the center within a plane that is perpendicular to the rotational direction that is to be detected, and at present the single-beam configuration is the chiefly used configuration.
However, the support of a single-beam configuration is difficult, it is difficult to achieve support that does not influence the resonator, and it is not possible to completely prevent leakage of vibration to the outside.
Examples of easy support that has been envisioned long ago are the four-beam tuning fork or the multi-beam tuning fork.
For example, in the Japanese Unexamined Patent Publication (KOKAI)No. 6-258083 there is disclosure of four-beam tuning fork vibrating gyro.
This four-beam tuning fork has a configuration having symmetry about the center within a plane that is perpendicular to the rotational direction that is to be detected, the same as with a single-beam configuration, and additionally, as a characteristic of the tuning fork configuration, because the bottom surface of the base part does not vibrate, it is possible to achieve complete vibrational isolation with the outside.
The four-beam vibrating gyro which is disclosed in the Japanese Unexamined Patent Publication (KOKAI)No. 6-258083 can be used to implement a vibrating gyro with almost no vibration of the base part, by selecting vibrational modes for which the directions of drive and Coriolis force detection are perpendicular from the six existing first-order vibration modes of the four-beam tuning fork and by using first order coupling thereof to detect Coriolis force.
The six first-order vibration modes of a four-beam tuning fork with good symmetry will now be described with reference to relevant accompanying drawings.
FIG. 21 is a front elevation view of a general type of four-beam tuning fork, in which the condition of the bottom surface of the base thereof being semi-fixed is indicated by hatching.
The sizes of various parts of this four-beam tuning fork are: overall length 4.8 mm, base length 1.92 mm, beam length 2.88 mm, base width 1.2 mm, beam width 0.48 mm, and groove 0.24 mm.
FIG. 22 through FIG. 27 are cross-sectional views of the beams as seen from the ends of the beams of the four-beam tuning fork, the six first-order vibration modes that each of the beams of this four-beam tuning fork having been calculated using the finite element method, verified by experiment, and indicated in sequence of increasing frequency.
Note, however, that the last torsion mode could not be verified by experiment.
FIG. 28 through FIG. 33 are cross-sectional view of the beams as seen from the ends of the beams of the same type of four-beam tuning fork, with a 1% reduction in the overall width of the tuning fork, but with no change in the thickness.
In contrast to FIG. 22 through FIG. 27, the cross-section of the beam is a non-square rectangle, and the six first-order vibration modes of each of the beams of this four-beam tuning fork as well were calculated using the finite element method, verified by experiment, and indicated in sequence of increasing frequency. In this case as well, it was not possible to verify the least torsion mode by experiment.
First, using FIG. 22 through FIG. 27, the vibration mode for the case in which the cross-section of the beams is a square will be described.
In FIG. 22, the arrows indicated in the drawing indicate the displacement direction of the beams at some given instant in time, and the vibration mode with these displacement directions will be called vibration mode 1, in which the centers of each of the beams are displaced so that their paths form a non-square rectangle, the characteristic vibration frequency thereof being 38.730 kHz.
In FIG. 23, the arrows in the drawing indicate the displacement direction of the beams at some given instant in time, and the vibration mode with these displacement directions will be called vibration mode 2, in which the paths of the centers of the four beams are displaced while maintaining a square shape, the characteristic vibration frequency thereof being 38.841 kHz.
In FIG. 24, the arrows indicate the displacement direction of the beams at some given instant in time, and this vibration mode will be called vibration mode 3, in which the paths of the centers of the four beams are displaced so as to form a diamond shape, the characteristic vibration frequency thereof being 39. 160 kHz.
In FIG. 25, the arrows indicate the displacement directions of the beams at some given instant in time, and this vibration will be called vibration mode 4, in which the paths of the center of the four beams are displaced mutually in parallel, the characteristic vibration frequency thereof being 39.483 kHz.
In FIG. 26, the arrows indicate the displacement directions of the beams at some given instant in time, and this vibration will be called vibration mode 5, in which the paths of the center of the four beams are displaced mutually in parallel, the characteristic vibration frequency thereof being 39.483 kHz.
In FIG. 27, the arrows indicate the displacement directions of the beams at some instant in time, and this vibration mode will be called vibration mode 6, in which the paths of the centers of the four beams are displaced so that the four-beam tuning fork is twisted, the characteristic vibration frequency thereof being 40.150 kHz. The reason that this mode 6 could not be verified by experiment was the strong oscillation of the base part.
Next, FIG. 28 through FIG. 33 will be used to describe the vibration modes for the case in which the cross-section of the beams is a non-square rectangle.
In FIG. 28, the arrows indicate the displacement direction of the beams at some given instant in time, and this vibration mode will be called mode 1, in which the paths of the centers of the four beams are displaced mutually in parallel, the characteristic vibration frequency thereof being 36.617 kHz.
In FIG. 29, the arrows indicate the displacement directions of the beams at some given instant in time, and this vibration mode will be called vibration mode 2, in which the paths of the centers of the four beams are displaced mutually in parallel, the characteristic vibration frequency thereof being 36.939 kHz.
In FIG. 30, the arrows indicate the displacement directions of the beams at some given instant in time, and this vibration mode will be called vibration mode 3, in which the paths of the centers of the four beams are displaced so as to form a diamond shape, the characteristic vibration frequency thereof being 37.099 kHz.
In FIG. 31, the arrows indicate the displacement directions of the beams at some given instant in time, and this vibration mode will be called vibration mode 4, in which the paths of the centers of the four beams are displaced mutually in parallel, the characteristic vibration frequency thereof being 37.256 kHz.
In FIG. 32, the arrows indicate the displacement directions of the beams at some given instant in time, and this vibration mode will be called vibration mode 5, in which the paths of the centers of the four beams are displaced mutually in parallel, the characteristic vibration frequency thereof being 37.608 kHz.
In FIG. 33, the arrows indicated the displacement directions of the beams at some given instant in time, and this vibration mode will be called vibration mode 6, in which the paths of the centers of the four beams are displaced so that the four-beam tuning fork is twisted, the characteristic vibration frequency thereof being 38.101 kHz.
Next, the association action will be described.
In the case of the four-beam tuning fork vibrating gyro that is disclosed in the Japanese Unexamined Patent Publication (KOKAI)No. 6-258083 of the six type of vibration mode of the four-beam tuning fork based on the vibration modes that exist for the case of a rectangular shape, a vibration mode for detection of the Coriolis excitation which is perpendicular to the driving vibration mode is selected, and the configuration for implementing this drive and detection are indicated.
In the first embodiment, the vibration mode 4 for a rectangular shape that is shown in FIG. 31 is taken as the driving vibration mode, and the vibration mode 5 for a rectangular shape that is shown in FIG. 32 is taken as the detection vibration mode. (Although it is not clearly indicated, it is not usual to select a mode with the lower characteristic vibration frequency as the detection vibration mode.)
In the second embodiment, the vibration mode 3 for a rectangular shape that is shown in FIG. 30 is taken as the driving vibration mode, and the vibration mode 1 for a square that is shown in FIG. 22, which does not exist for the case of a rectangular shape, is taken as the detection vibration mode, and a configuration for implementing this driving and detection is indicated.
In the third embodiment, a method is indicated for detecting the vibration mode for a rectangular shape that is shown in FIG. 28 or the vibration mode 2 for a rectangular shape that is shown in FIG. 29 from the first-order coupling of the vibration mode 3 for a rectangular shape that is shown in FIG. 30 and the vibration mode 1 for a square that excites the Coriolis force that is shown in FIG. 22, and a configuration for implementing this driving and detection is indicated.
However, the following problems exists with the four-beam tuning fork that is disclosed in the Japanese Unexamined Patent Publication (KOKAI)No. 6-258083.
First, in the first embodiment, with the vibration mode 4 for a rectangular shape that is shown in FIG. 31 and the vibration mode 5 for a rectangular shape that is shown in FIG. 32, because of the difference between the characteristic vibration frequencies it is not possible to achieve a large detection sensitivity due to the lack of sufficient excitation of vibration mode 5 by vibration mode 4.
With regard to this point, while the Japanese Unexamined Patent Publication (KOKAI)No. 6-258083 has language to the effect of using symmetry, that is, of using a square shape, in actuality there is no vibration mode such as vibration mode 4 and vibration mode 5 for a rectangular shape that are shown in FIG. 31 and FIG. 32 for the case of a square, and the vibration mode such as vibration mode 4 and vibration mode 5 appear for a square, as shown in FIG. 25 and FIG. 26.
Experimentally, if the difference in the characteristic vibration frequencies for the two directions approaches approximately 100,000 ppm, coupling already causes the rectangular vibration mode 4 and vibration mode 5 shown in FIG. 31 and FIG. 32 to cease to exist.
Therefore, the first embodiment which is disclosed in the Japanese Unexamined Patent Publication (KOKAI)No. 6-258083 is either implemented using a non-resonant four-beam tuning fork in which the frequency difference is more than 100,000 ppm, or the by a resonant type which, even without Coriolis force, detects Coriolis force with an extremely high output being made.
In the case of a non-resonant type, because the Coriolis force detection sensitivity will be poor, this will result in a worsened signal-to-noise (S/N) ratio for Coriolis force detection, and in the case of a resonant type it is necessary to detect a Coriolis force from an output that is much larger than the output that is caused by the Coriolis force, this forcing the measurement to be performed with an extremely wide dynamic range, which is disadvantageous from the standpoint of achieving a high S/N ratio.
Additionally, while there is a proposal of a mechanism to limit the output by using a closed loop, this does not change the S/N ratio.
Turning next to the remaining embodiments that are disclosed in the Japanese Unexamined Patent Publication (KOKAI)No. 6-253083, the rectangular vibration mode 5 shown in FIG. 30 is used for driving, and the rectangular vibration mode 1 or mode 2 shown in FIG. 27 and FIG. 28, respectively, or a the square vibration mode 1 which is shown in FIG. 22 and which is generated from the coupling therebetween is used for detection.
In the case of a rectangular shape, if the vibration mode 6, for which detection is not possible, shown in FIG. 33, is eliminated, the vibration mode 3 which is shown in FIG. 30 is the only mode with coincides with the square.
There is a clear difference in characteristic vibration frequency between this and the vibration mode for detection.
With regard to this frequency difference, if one considers the vibration modes which are intrinsically different, even if it is possible to perform adjustment so that the characteristic vibration frequencies coincide, this would affect the overall symmetry of the tuning fork, thereby increasing the vibration noise, making it impossible to achieve a high Coriolis force detection S/N ratio.
An object of the present invention is to provide a vibrating gyro which solves the above-described problems, this vibrating gyro having a high detection sensitivity and good detection accuracy, without influence from the method of support thereof.